Mitoxantrone is an anti-cancer agent used in the treatment of breast and prostate cancers. It is classified as a topoisomerase II poison, however can also be activated by formaldehyde to generate drug–DNA adducts. Despite identification of this novel form of mitoxantrone–DNA interaction, excessively high, biologically irrelevant drug concentrations are necessary to generate adducts. A search for mitoxantrone analogues that could potentially undergo this reaction with DNA more efficiently identified Pixantrone as an ideal candidate. An in vitro crosslinking assay demonstrated that Pixantrone is efficiently activated by formaldehyde to generate covalent drug–DNA adducts capable of stabilizing double-stranded DNA in denaturing conditions. Pixantrone–DNA adduct formation is both concentration and time dependent and the reaction exhibits an absolute requirement for formaldehyde. In a direct comparison with mitoxantrone–DNA adduct formation, Pixantrone exhibited a 10- to 100-fold greater propensity to generate adducts at equimolar formaldehyde and drug concentrations. Pixantrone–DNA adducts are thermally and temporally labile, yet they exhibit a greater thermal midpoint temperature and an extended half-life at 37°C when compared to mitoxantrone–DNA adducts. Unlike mitoxantrone, this enhanced stability, coupled with a greater propensity to form covalent drug–DNA adducts, may endow formaldehyde-activated Pixantrone with the attributes required for Pixantrone–DNA adducts to be biologically active.
When mitoxantrone is activated by formaldehyde it can form adducts with DNA. These occur preferentially at CpG and CpA sequences and are enhanced 2-3-fold at methylated CpG sequences compared with non-methylated sites. We sought to understand the molecular factors involved in enhanced adduct formation at these methylated sites. This required, first, clarification of factors that contributed to the formation of adducts at CpG sites. For this purpose mass spectrometry of an oligonucleotide duplex (containing a single CpG adduct site) was used to confirm the presence of an additional carbon atom (derived from formaldehyde) on the drug-DNA complex. The effect of 3-flanking sequences was revealed by electrophoretic analysis of oligonucleotidedrug adducts, and the preferred adduct-forming site was identified as 5-CGG-3. Radiolabeled studies of drug-DNA adducts confirmed that the site of attachment involved the exocyclic amino of guanine. Molecular modeling analysis of the relative stability of the intercalated form of mitoxantrone was consistent with observed adduct-forming potential of CG sites with varying flanking sequences. The known preference for adduct formation at methylated CG sites was confirmed by energetics calculations and shown to be due to a shift of equilibrium of the intercalated form of the drug from the major groove (at CG sites) to the minor groove (at methylated CG sites). This increases the relative amount of drug that is located adjacent to the N-2 exocyclic amino of guanine in the minor groove, where covalent linkage is facilitated. These results account for the enhanced covalent binding of mitoxantrone to methylated CG sequences and provide a molecular model of the interactions.
The DNA binding of the dinuclear platinum complex trans-[{Pt(NH 3 ) 2 Cl} 2 µ-dpzm] 2ϩ (di-Pt), linked with the 4,4Ј-dipyrazolylmethane (dpzm) linker, was examined by 1 H and 195 Pt NMR and transcription assays. At 60 ЊC, di-Pt reacts with guanosine two-fold slower (t 1/2 : 1 h) than cisplatin (t 1/2 : 0.5 h). With adenosine, the di-Pt complex reacts much slower (t 1/2 : 7 h) forming a range of different adducts through the N7 and either the N1 or N3 positions of the nucleoside. From 1 H NMR analysis of the major product of the reaction of di-Pt with the oligonucleotide d(ATGCAT) 2 , purified by HPLC, it was determined that the dinuclear platinum complex can readily form a 1,2-GG interstrand DNA cross-link. Transcription assays using di-Pt and the lac UV5 promoter indicated that the metal complex forms an array of adducts vastly different from cisplatin. The two greatest blockages occurred at adenine residues, with possible interstrand and intrastrand AA and AG adducts being formed. These results indicate that unlike other platinum based anti-cancer agents, di-Pt binds to DNA with a preference for adenine bases.
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